Question

Consider a full Moon rising precisely at sunset on a certain day. Approximately when will the Moon rise on the next day?

choices

About an hour before sunset

   

About an hour after sunset

   

About the same time as sunset

Answer 1

About an hour after sunset.

The Moon is continually moving on ahead in its orbit while the Earth rotates. So 24 hours later, the Earth has rotated back around to the same place it was the night before, but the Moon has gone on ahead. Think of it like the second hand on an analog watch; it’s going around the face of the clock much faster than the minute hand, but each time the second hand goes around, the minute hand has moved, and so it takes an extra second to line back up with the minute hand. Because the Moon has moved 13 degrees or so since its last moonrise, it’s going to take another hour or so for the Earth to catch back up to the Moon’s new location, delaying the Moon's rising above your horizons by ~50 minutes each day.